By carbon nanotubes are understood principally cylindrical carbon tubes with a diameter between 3 and 80 nm, the length being a multiple, at least 10-fold, of the diameter. These tubes consist of layers of ordered carbon atoms and have a nucleus different in morphology. These carbon nanotubes are also referred to for example as “carbon fibrils” or “hollow carbon fibres” or “bamboo”. Because of their dimensions and their particular properties, the carbon nanotubes described have an industrial importance for the production of composite materials. Other substantial possibilities are in electronics, energy and other applications.
Carbon nanotubes are a material known for some time. Although Iijima in 1991 (S. Iijima, Nature 354, 56-58, 1991) is generally referred to as the discoverer of nanotubes, these materials, in particular fibrous graphite materials with several graphite layers, have been known even longer.
The known methods for the production of carbon nanotubes include for example arc, laser ablation and catalytic processes. In many of these processes, carbon black, amorphous carbon and fibres with high diameters are formed as by-products. In the catalytic processes, a distinction can be made between deposition on supported catalyst particles and deposition on metal centres formed in situ with diameters in the nanometer range (so-called “flow process”). In production by the catalytic deposition of carbon from hydrocarbons gaseous under reaction conditions (hereafter CCVD—catalytic chemical vapour deposition), acetylene, methane, ethane, ethylene, butane, butene, butadiene, benzene and other educts containing carbon are named as possible carbon donors. For example, the production of carbon nanotubes by the decomposition of light (i.e. short- and medium-chain aliphatic or one- or two-nucleus aromatic) hydrocarbons on a catalyst based on iron at temperatures above 800-900° C. are described in EP 205 556 B1 and WO A 86/03455.
The catalysts usually used in the prior art (De Jong et. al. Catal. Rev.-Sci. Eng., 42(4), 481-510, 2000) as a rule contain metals, metal oxides or decomposable or reducible metal components such as e.g. Fe, Mo, Ni, V, Mn, Sn, Co, Cu and others.
The formation of carbon nanotubes and the properties of the tubes formed depend in a complex way on the metal component used as catalyst or a combination of several metal components, the support material used and the interaction between catalyst and support, the production method of the catalyst, the educt gas and partial pressure, an admixture of hydrogen or other gases, the reaction temperature and the dwell time or the reactor used. An optimisation is a particular challenge for an industrial process.
It should be noted that the metal component used in CCVD and referred to as catalyst is consumed in the course of the synthesis process. This consumption can be attributed to a deactivation of the metal component, e.g. because of deposition of carbon on the whole particle which leads to the complete covering of the particle (this is known to the skilled person as “encapping”). As a rule, a reactivation is not possible or economically not meaningful. Often only a maximum of a few grams of carbon nanotubes per gram of catalyst are obtained, the catalyst here comprising the whole of the support and catalytically active materials used. Because of the consumption of catalyst described, a high yield of carbon nanotubes based on the catalyst used is an essential requirement for catalyst and process.
For an industrial production of carbon nanotubes, e.g. as a constituent for improving the mechanical properties or conductivity of composite materials, as with all industrial processes a high space-time yield is to be strived for in obtaining the particular properties of the nanotubes and minimising the energy and materials to be used.
In EP 0205 556 A1 (Hyperion Catalysis International) is described the production of carbon nanotubes on iron-containing catalysts supported on alumina which have been produced by means of incipient wetness. The carbon nanotubes produced have, at 10-45 nm, a very broad distribution of the outer carbon nanotube diameters. The production of Ni supported catalysts (γ-Al2O3) has been described e.g. in the dissertation by M. G. Nijkamp, Universiteit Utrecht, NL, 2002 “Hydrogen Storage using Physisorption Modified Carbon Nanofibers and Related Materials”. Ni-based systems are also described by Shaikhutdinov et al. (Shamil' K. Shaikhutdinov, L. B. Avdeeva, O. V. Goncharova, D. I. Kochubey, B. N. Novgorodov, L. M. Plyasova, “Coprecipitated Ni—Al and Ni—Cu—Al catalysts for methane decomposition and carbon deposition I.”, Applied Catalysis A: General, 126, 1995, pages 125-139) as active in the decomposition of methane to carbon nanomaterials. These catalysts were produced by a discontinuous precipitation.
Catalyst systems with a high level of catalytically active metal components—up to 100 wt. %—with at the same time extremely low diameters of the metalite centres, have recently been developed to maximise the space-time yield. Catalysts of this type are the “full contacts” generally known to the skilled person. A catalyst of this type is disclosed in DE-A 10 2004 054 959. This catalyst produced discontinuously in a stirred tank by coprecipitation of the corresponding metal salts is distinguished by a high productivity. The carbon nanotubes produced, however, have a very broad distribution of the geometric measurements (for example, external diameter: from 5 to approx. 40 nm). The reason for this is probably to be found in the effect on the catalyst properties by conditions during discontinuous precipitation. Discontinuous precipitation in a stirred tank has the disadvantage that microbe formation and microbe growth takes place during the entire addition of the precipitation agent. A catalyst which has a broad size distribution of the primary particles and at the same time a broad distribution of the catalytically active metalite centres which leads to carbon nanotubes with a broad distribution of the geometric measurements (for example distribution of inner/outer diameter, number of carbon layers, length of the carbon nanotubes, layer arrangement etc.) is obtained by this method. This distribution has a crucial effect on the application properties—dispersion in polymers, electrical and mechanical properties etc.—and consequently the commercial use of the carbon nanotubes. Consequently it is clear that apart from the high productivity, an extremely narrow distribution of the catalytically active metalite centres have a crucial industrial importance.
In principle, the catalysts used as prior art have the disadvantage that they have too low a productivity and/or the carbon nanotubes formed therefrom have too broad a distribution of the geometric measurements.
The object of the present invention was therefore, based on the prior art, to provide a catalyst which is distinguished by a high space-time yield and by a narrow distribution of the geometric measurements of the multi-layer carbon nanotubes produced in the catalytic decomposition of carbon-containing educt gases.